1. Field of the Invention
The present invention relates to diffractive optical elements, and more particularly to diffractive optical elements having a blazed-binary grating, as well as optical systems and optical apparatuses provided with the same.
2. Description of the Related Art
Conventional diffractive optical elements serving as diffraction lenses with the purpose of reducing chromatic aberration are disclosed in SPIE Vol. 1354 International Lens Design Conference (1990), for example.
Color separation gratings utilizing the fact that the diffraction angle differs depending on the wavelength of the incident light in order to perform color separation are disclosed for example in Japanese Patent Publication No. 1993-46139 (corresponding to U.S. Pat. No. 5,113,067). Furthermore, recently, a diffractive optical element known as SWS (sub-wavelength structure) grating, whose grating period has a microscopic periodic structure that is smaller than the used wavelength, has been disclosed in “Kougaku” [Optics], vol. 1 of 27 in series (1998), pp. 12 to 17.
Due to its grating structure, this SWS grating is used for elements with various functions, such as birefringent wavelength plates, anti-reflection structures and polarization beam splitters. Moreover, it has been reported that for these functions, there are only small performance variations due to changes of the angle of incidence.
Among SWS gratings, Applied Optics, Vol. 31, No. 22, p. 4453 (1992) discloses the structure shown in FIG. 10 as a diffractive optical element known as a blazed binary grating.
In the diffractive optical element of FIG. 10, a blazed-binary grating 13 is formed on a substrate 12. The blazed-binary grating 13 is a SWS grating in which one-dimensional rectangular gratings are fabricated with a period p1 that is smaller than the wavelength of the incident light (used wavelength). The SWS rectangular grating is formed at the border between a region 14 of a first material and a region 15 of a second material. When the grating width of the first material is wi (i=1 . . . s), then the proportion fi (=wi/p1, i=1 . . . s) that the first material occupies within the grating pitch repeatedly changes gradually from f1 to fs within a period Pt that is larger than the wavelength of the incident light.
With this structure, the effective refractive index changes gradually even though the grating height (depth of the grating grooves) is constant, and as a result, it is possible to attain a performance that is substantially the same as that of a blazed diffraction grating with a constant refractive index in which the height of the grating portions 10 changes gradually, as shown in FIG. 14.
FIG. 11 shows the diffraction efficiency as a function of the wavelength of polarized waves in which the electric field component is parallel to the blazed binary grating grooves (referred to as “TE polarized light” in the following).
Moreover, recently, the assignee of this application has proposed, as the structure of a diffractive optical element, the diffractive optical element shown in FIG. 12. In this diffractive optical element, the grating thickness (grating height) of two grating portions 10′ and 11′ made of different materials is changed gradually (monotonously), and by stacking the two gratings 10′ and 11′ on top of one another in the thickness (height) direction, a high diffraction efficiency in the entire visible wavelength region can be achieved, as shown in FIG. 13.
In these conventional examples, the structure of blazed-binary gratings has come to a stand-still at obtaining the same performance as the blazed-binary grating shown in FIG. 14, and there are limits to using them in the entire visible wavelength region.